A normal human wrist may be considered as comprising three sets of bones: the distal forearm, constituting the distal portion of the radius and the ulna; the carpals, constituting eight bones divided into two rows, i.e. the proximal bones (scaphoid, lunate, triquetrum, and pisiform) and the distal bones (trapezium, trapezoid, capitate, and hamate), that are most closely associated with the motion of the wrist; and the metacarpals, constituting the distal segments (i.e. thumb and four fingers).
The wrist is commonly considered a biaxial joint, meaning that there are two principle movements of the wrist, namely an extension-flexion movement and a radial/ulnar movement. Although the wrist has no intrinsic mechanism for active supination/pronation deviation movement, it is currently thought that there is likely some degree of passive motion associated with a torsional force transmitted across the radial-carpal joint. While various wrist prosthetics have been developed and patented, they all suffer from loosening of one of the two components of the wrist prosthetic. The torsional loads cannot be passed onto soft tissue due to the constrained design of prostheses. The torsional loads combined with media ulnar and radial deviation causing off center loads can lead to a “window-wiper” action of the central stem of the metacarpal component against the dorsal aspect of the middle metacarpal.
Recognition of such passive torsional forces has led to various wrist prosthetic designs that attempt to compensate for such passive torsional forces. These designs attempt to provide a more stable fixation. One type of stable fixation design that attempts to compensate for passive torsional forces adds rotational control pegs to a distal component of the wrist prosthetic. Another type of stable fixation design relies on screw-type fixation of a metacarpal component. Such designs have not been well received due to the inherently weak bone stock available for the metacarpal component in typical wrist implant patients. Also, some designs fail because there is an effort to obtain greater fixation, when motion is still present.
Another manner of attempting to compensate for such torsional forces is mismatching of wrist components. Particularly, a surgeon may match small metacarpal components with larger radial components. This, however, provides a less conforming articulating surface, thus allowing for greater contact stresses and greater potential for dislocation.
Another problem with wrist prosthetics is loosening of the distal implant component after implantation. In an attempt to solve this problem, various solutions have been proposed. These solutions, however, typically involve the creation of more and/or longer stems which are intended to penetrate deeper into the second, third, and fourth metacarpal canals. While at a first glance this solution seems probable to solve the problem. This solution, however, has not produced superior results to the primary implant outcome.
It would thus be advantageous to provide a distal component for a wrist prosthesis that overcomes one or more of the disadvantages of the prior art.